Complex hydrides such as sodium aluminum hydrides (i.e., NaAlH4), like many other alanates of alkaline metals, are promising materials for hydrogen storage because of their relatively large capacity for storage of useful hydrogen. For example, NaAlH4 has a hydrogen storage capacity of 5.6 wt %. The main problem of these complex hydride materials, however, is their slow hydrogenation and dehydrogenation kinetics. In pure sodium alanates, dehydrogenation temperatures take place well above 200° C. despite the fact that the reactions as set forth below are thermodynamically favorable:3NaAlH4→Na3AlH6+2Al+3H2  (1)Na3AlH6→3NaH+Al+3/2H2  (2)
The strong catalyst behavior of Ti on NaAlH4 was first noticed using solution chemistry techniques, whereby nonaqueous solutions of NaAlH4 and either TiCl3 or Ti(OBun)4 catalyst precursors were decomposed to solid Ti-doped NaAlH4. This work was referenced in the publication by B. Bodanovic, and M. Schwickardi, J. Alloys Comp. 253 (1997) 1, and which is incorporated herein by reference.
The existence of some synergistic interaction between Ti and other metal catalysts such as Fe and Ni when doped in alanates is also known. Such interactions have been reported in the publications of B. Bogdanovic, R. A. Brand, A. Marjanovic, M. Schwickardi, and J. Tölle, J. Alloys Comp. 302 (2000) 36; R. A. Zidan, S. Takara, A. G. Hee C. M. Jensen, J Alloys Comp. 285 (1999) 119; and, C. M. Jensen, R. Zidan, N. Mariels, A. Hee, C. Hagen, Inter. J. Hydrogen Energy 24 (1999) 461; the above publications being incorporated herein by reference.
The Zidan et al publication referenced above also sets forth that a further lowering of the dehydrogenation temperature depends largely on the doping and homogenization procedures and also noted the important catalytic role of Zr. Today, there is general agreement that one of the main dynamic mechanisms controlling the adsorption/absorption and desorption of H2 is a diffusional one and thus the kinetic behavior of both processes depends largely on the characteristic particle size of the samples. Because of this, high intensity ball milling has become the most important if not the only procedure for preparing and doping alanates.
Another negative aspect of alanates is the strong deleterious effect that repeated cycles of hydrogen adsorption and desorption has on both kinetic performance and hydrogen capacity in these materials. The Bogdanovic et al. publication (J. Alloy Compd. 350 (2003) 246 and incorporated herein by reference) suggested that the latter resulted from an incomplete reaction between phases of Na3AlH6 and Al, which become isolated by regenerated NaAlH4 that is formed. The data in Bogdanovic et al convincingly demonstrated that adding small but proper amounts of Al metal to the alanates prior to ball milling solves the problem.
The development of catalyzed alanates having better performance kinetics has not progressed and is largely attributable to the limited techniques, such as ball milling, which are available. This lack of advancement in the field suggests the necessity for different approaches. One such approach is the utilization of graphites as additives to alanates.
By mechanisms that are not yet fully understood, it has recently been discovered that graphitic structures, such as fullerenes, diverse graphites and even carbon nanotubes, play, under certain conditions, an important role in improving the kinetics of dehydrogenation and reversibility of certain complex metal hydrides. As defined herein a “complex metal hydride” refers to AlH3-based hydrides such as NaAlH4 which readily liberate hydrogen at moderate temperatures on the order of about 100° C. to about 150° C. and which yields a dehydrogenated form of hydride which cannot easily be regenerated with hydrogen gas or which requires extreme conditions in order to rehydrate. Accordingly, complex metal hydrides should not be confused with metal, intermetallics, or alloyed hydrides such as but not limited to MgH2, LaNi5H6, or FeTiH1.2 respectively.
A series of studies have shown that Mg2Ni experiences marked desorption and capacity improvements when mixed and milled together with graphitic carbon. See the publications of S. Ruggeri, L. Roué, G. Liang, et al., J. Alloy Compd. 343 (2002) 170; C. Iwakura, H. Inoue, S. G. Zhang, et al., J. Alloy Compd. 293-295 (1999) 653; and S. Bouaricha, J. P. Dodelet, D. Guay, et al., J. Alloy Compd. 325 (2001) 245, which publications are incorporated herein by reference. Zaluska et al. has investigated the role of carbon on alanates and demonstrated that carbon improves the dehydrogenation and hydrogenation kinetics of sodium analates as seen in the publication A. Zaluska, L. Zaluski, and J. O. Ström-Olsen, J. Alloy Compd. 298 (2000) 125 which is incorporated herein by reference. However, results of this work indicate that graphite does not improve the kinetics. In addition, no study has yet investigated the effects of carbon, including graphite, activated carbon, or even carbon nanotubes, upon samples of Ti-doped NaAlH4 or other catalyst dopant.
U.S. Pat. No. 6,680,042, assigned to Hydro-Quebec, discusses the use of graphite along with various liquid hydrocarbons which are beneficial for overcoming an oxide coating on the surface of a hydrogen storage material. In the case of magnesium metal, the presence of graphite reduces the time required to prepare a hydride when the hydride is subjected to intense mechanical grinding under hydrogen pressure and at high temperatures. The reference does not report any improvements in desorption kinetics for materials prepared with graphite nor does the reference discuss the use of titanium doped NaAlH4 materials.
Accordingly, there remains room for improvement and variation within the art.